Photocells and associated circuitry (photo sensor) for converting light levels into an electrical signal are known. Photo sensors are used, for example, to control street lights and the like such that the lights are turned off during day time and on at night.
Photo sensors and specifically photocells come in various technologies, e.g., Silicon and Cadmium Sulphide. Cadmium Sulphide based sensors or cells are typically available as light dependent resistors that advantageously resemble a human eye response to light, however suffer from an irreversible drift with temperature and thus have not been used in situations that require repeated operation, i.e., light detection. Silicon based sensors or cells are available as photo diodes and photo transistors. Photo transistors typically demonstrate better sensitivity with larger output signals available for a given light input. Silicon sensors also have a long life expectancy (tens of years) which is typically required or very desirable in most lighting situations.
Silicon cells or sensors while having long life expectancies and good sensitivities and being available in small sizes have various problems. For example, these cells are known to vary with temperature and from one cell to another. These variations are not acceptable in some situations where accurate light levels are required. One technique, for overcoming variations due to temperature is to control the temperature of the photo cell, however controlling temperature comes with its own set of difficulties, e.g., hardware costs in terms of size and economics.